Paleoclimate Proxies

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Paleoclimate proxies are physical, chemical and biological materials preserved within the geologic record (in paleoclimate archives) that can be analyzed and correlated with climate or environmental parameters in the modern world. Scientists combine proxy-based paleoclimate reconstructions with instrumental records (such as thermometer and rain gauge readings) to expand our understanding of climate variability to times before humans began measuring these things. These reconstructions of past climate and environment span all timescales, from year-to-year variations to those that occurred over millions of years. These data help us understand how the Earth's climate system varied both before and after human alteration of the landscape.

The use of a proxy to reconstruct past climate requires an understanding of how that proxy is related to some aspect of climate. For example, some proxies, such as atmospheric gases trapped in glacial ice (e.g., carbon dioxide and methane), provide a relatively direct measurement of atmospheric chemistry at the time the ice formed and was sealed off from the atmosphere. Other proxies are less direct, such as stable isotope measurements (e.g., oxygen and carbon) from shells of marine organisms. These indirect proxies require calibration studies in the modern system to establish the relationship between climate processes and the proxy.

Physical proxies

Physical proxies include characteristics such as sediment composition, texture, structure, color, density, and magnetic properties, among others. Scientists use changes in physical properties of archive materials to infer past climate conditions. Environmental change, driven by climate, human activity, or other factors, can alter the physical properties of sediments in predictable ways. Understanding these relationships provides a means to reconstruct the climate conditions at the time the sediments were deposited.

Biological proxies

Biological proxies include remains of living organisms, such as pollen, foraminifera (single-celled, microscopic organisms that bear an external chambered shell), mollusks, and ostracodes (small members of the Crustacean (shrimp) family that are encased by two shells). Because the distribution of these organisms is controlled by temperature, moisture availability, and other environmental factors, their presence in a sample allows scientists to make inferences about the climate when the sample was deposited. Some examples of biological proxies are shown below.

Chemical Proxies

The chemical composition of shells of aquatic organisms is affected by the chemistry of the water in which they form. Water, in turn, is influenced by temperature and precipitation. Consequently, shell chemistry (stable isotope and elemental composition) is one example of a chemical proxy of temperature and precipitation. For organisms such as corals and mollusks that secrete sequential layers, these layers can provide an archive of change over the lifespan of the animal. Organic biomarkers are another type of chemical proxy; these are molecular fossils derived from living organisms (such as plants), and they can serve as proxies for other physical and chemical properties of their environment (e.g., temperature, pH, salinity).

The United States Geological Survey

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